The assessment date varies depending on the tax and when you file the return. For example, if you filed your 2009 tax return on March 1, 2010 (before the April 15 deadline) the assessment date would be April 15, 2010 since that is the due date. But, if you filed the return late, the assessment date would be sometime after the IRS has reviewed the return and accepted it. Probably in the neighborhood of 8 weeks from the time you filed the return. I would suggest that you obtain a copy of the statement of account for the years that you owe for and there you can find out exactly when the IRS assessed the taxes against you and can estimate when the statute of limitations will expire.